trait type to distinguish generic offset array and one based indexing array

[johnnychen94]

Traits like this might be helpful to be kept in ArrayInterfaceCore

struct GenericIndexing end
struct OneBasedIndexing end

offset_type(::Array) = OneBasedIndexing()
offset_type(::OffsetArray) = GenericIndexing()
offset_type(A::Diagonal) = offset_type(parent(A))

CRef: https://discourse.julialang.org/t/does-abstractarray-itself-need-to-support-indexing-beyond-1-based/83324/3

[Tokazama]

If you need something right away you could probably accomplish this by doing is_offset_from_parent(T::Type) = any(!=(1), known_offsets(parent_type(T))), since the only time known_offsets doesn't return ones is when dealing with non one-based indexing.

In general, it would be helpful to have more tools that assist in transforming between index states so that when wrapper like Diagonal defines its own indexing rules we know what is safe/optimal to pass off to the parent array.

[johnnychen94]

I started to be skeptical about this proposal now as it only provides a static alternative to the current Base.require_one_based_indexing. And the benefit seems doesn't worth the efforts. -- I somehow believe Julia compiler could just optimize this runtime axes check away.

I've outlined the reasoning in the discourse discussion https://discourse.julialang.org/t/does-abstractarray-itself-need-to-support-indexing-beyond-1-based/83324/6?u=johnnychen94

[Tokazama]

As you mentioned in that comment, all you really want to do is ensure the first value of an axis is 1 without any overhead. Using known_offsets should facilitate this since it throws everything into the type domain.

[johnnychen94]

Agreed, let me close this for the moment, we can re-discuss if there's someone else finding it something must-to-have.